Miniature microphones, which may be used in various applications (e.g., cellular phones, laptop computers, portable consumer electronics, hearing aids), typically include a membrane and a rigid back electrode in close proximity to form a capacitor with a gap. Incoming sound induces vibrations in the compliant membrane and these vibrations change the capacitance of the structure which can be sensed with electronics. Typically, the structure of the microphone contains a large backside cavity and a small pressure release hole. The pressure release hole allows the large atmospheric pressure to reach the backside of the membrane. While the membrane compliance is designed to resolve dynamic pressure vibrations with magnitudes of 1 μPa to 1 Pa, atmospheric pressure is approximately 100 kPa (about a factor of 105 times larger). Without a pressure release, it is challenging to design compliant membranes that do not collapse under atmospheric pressure.
Recently, microelectromechanical systems (MEMS) processing has been utilized to fabricate miniature microphones. However, most miniature microphones using MEMS processing use a deep reactive ion etch step through the entire silicon substrate, thereby preventing CMOS compatibility. If, however, miniature microphones could use MEMS processing without the use of the deep reactive ion etch step, then miniature microphones could be manufactured with CMOS compatible processes which have a significant cost advantage over other processes.
Furthermore, there is a desire to create a vacuum sealed microphone. By removing air from the gap, a microphone with much lower self-noise (which results in higher fidelity) can be fabricated with a potentially better frequency response. However, a very stiff diaphragm would be required to prevent the structure from collapsing under external atmospheric pressure, and such a structure would have poor sensitivity to small sound pressure due to its stiffness. Such structures have been manufactured using MEMS processing to realize ultrasound sensors but not functional microphones.